In Code Division Multiple Access (“CDMA”) wireless communication, one carrier frequency bandwidth is used for many mobile stations. Mobile stations are able to distinguish their calls from those intended for other stations because the various channels are code spaced. In other words the signals are coded to be orthogonal to each other. Each mobile station attempts to align its receiver with their signal. Thus, only the receiver properly aligned with the unique, user-specific code will receive the call intended for its station. The other calls will appear as noise to that station.
In order for a mobile station to detect a call intended for it, the mobile station must be able to distinguish the channel carrying the call from noise. This requires the transmitted signal power of its call to be at, or above, some power level. Traditionally, radio frequency (“RF”) amplifiers are used in a CDMA base station to boost the radio signal so that it arrives at the mobile station at a power level 10 to 15 dB below the noise floor. The coding gain in the mobile station receiver brings the signal to a recognizable level.
The radio and RF amplifier of a base station must be designed with a certain amount of headroom above the maximum average voltage level to handle signal peaks above the maximum average level. More specifically, headroom is the linear amplification range defined by the top of the linear range of the signal, divided by the average of the full power signal expressed in dB units. Typically, in a base station, radios are designed with 11 to 13 dB of headroom and RF amplifiers are designed with 7 to 9 dB of headroom.
The amount of headroom required by the radio and RF amplifier effects the cost and efficiency of the base station. It dictates the size of the RF amplifier which in turn impacts the size of the power supplies, the required cooling capacity, cabinet size and weight and noise. By reducing the headroom of the radio and RF amplifier the operational costs of the base station can be reduced proportionally. Historically, the required headroom for the radio and RE amplifier is constrained by controlling the peak to average ratio of the signal. Conventionally this is accomplished in a peak to root mean square (“rms”) reducer, which can implement one of a variety of methods to constrain the peak signal values.
One known method of reducing peak-to average ratios is to hard limit signal peaks. In other words, all peak values exceeding a specified limit are set to the value of that limit. A second approach is to simply eliminate the peak energy from the baseband signal whenever the peak exceeds the specified limit. Yet a third approach is to add an inverse sinc function to lower the amplitude of a peak signal to a level below the constraining limit. Similar to the third approach, a fourth method to reduce the peak to average ratio multiplies a scaling function to all peak signals above the constraining limit. The scaling function is selected so as to lower the peak signals below the constraining limit.
All of the above-mentioned methods focus on reducing the peak values while leaving the average signal values virtually untouched. While these methods are known and used in the art, those skilled in the art continue to seek new methods to further reduce the amount of headroom required by the radio and RF amplifier so as to improve efficiency. Furthermore, none of the prior art methods address the efficient use of the digital to analog converter (“DAC”) that precedes the radio and RF amplifier. As peak signals are reduced DACs are being underutilized.